This invention relates to highly efficient processes for the preparation of tetrabromobisphenol-A.
Tetrabromobisphenol-A is one of the most widely used brominated flame retardants in the world. It is used extensively to provide flame retardency for styrenic thermoplastics and for some thermoset resins.
The commercial processes used to produce tetrabromobisphenol-A generally fall into three categories. The first category includes those processes in which substantial amounts of methyl bromide are co-produced along with the tetrabromobisphenol-A. Generally, up to 40-50 pounds of methyl bromide can be expected per 100 pounds of tetrabromobisphenol-A produced. The methyl bromide co-production is now considered desirable since there is a substantial market for this bromide as a fumigant and as a pharmaceutical or agricultural chemical intermediate. In most cases, the processes within this first category feature reacting bisphenol-A and bromine in methanol. The ar-bromination of the bisphenol-A is a substitution reaction which generates one mole of HBr per ar-bromination site. Thus, for the production of tetrabromobisphenol-A, four moles of HBr are generated per mole of tetrabromobisphenol-A produced. The HBr in turn reacts with the methanol solvent to produce the methyl bromide co-product. After the bisphenol-A and bromine feed are finished, the reactor contents are cooked for one to two hours to complete the reaction. At the end of the reaction, water is added to the reactor contents to precipitate out the desired tetrabromobisphenol-A product.
The second category of processes features the production of tetrabromobisphenol-A without the co-production of substantial amounts of methyl bromide and without the use of oxidants to convert the HBr to Br.sub.2. See U.S. Pat. No. 4,990,321; U.S. Pat. No. 5,008,469; U.S. Pat. No. 5,059,726; and U.S. Pat. No. 5,138,103. Generally, these processes brominate the bisphenol-A at a low temperature, say 0.degree. to 20.degree. C., in the presence of a methanol solvent and a specified amount of water. The water and low temperature attenuate the production of methyl bromide by slowing the reaction between methanol and HBr. The amount of water used, however, is not so large as to cause the precipitation of the tetrabromobisphenol-A from the reaction mass. Additional water for that purpose is added at the end of the reaction. One drawback with this type of process is that it uses a fairly long aging or cook period after the reactants have all been fed and it requires additional process time for the final precipitation of tetrabromobisphenol-A via the last water addition.
In the third category are those processes which feature the bromination of bisphenol-A with bromine in the presence of a solvent and, optionally, an oxidant, e.g., H.sub.2 O.sub.2, Cl.sub.2, etc. See U.S. Pat. No. 3,929,907; U.S. Pat. No. 4,180,684; U.S. Pat. No. 5,068,463 and Japanese 77034620 B4 77/09/05. The solvent is generally a water immiscible halogenated organic compound. Water is used in the reaction mass to provide a two-phase system. As the bisphenol-A is brominated, the tetrabromobisphenol-A is found in the solvent. The co-produced HBr is present in the water. When used, the oxidant oxidizes the HBr to Br.sub.2, which in turn is then available to brominate more bisphenol-A and its under-brominated species. By oxidizing the HBr to Br.sub.2, only about two moles of Br.sub.2 feed are needed per mole of bisphenol-A fed to the reactor. To recover the tetrabromobisphenol-A from the solvent, the solution is cooled until tetrabromobisphenol-A precipitation occurs. This process type is not a panacea though, as there is the expense of handling, purifying and recycling the halogenated organic solvent. In addition, the cooling of the solution to recover tetrabromobisphenol-A entails additional expense and process time.
As long as there is a viable market for methyl bromide, the processes of the first category have been found to be commercially attractive. However, it is now being proposed, on an international level, that the use of methyl bromide as a fumigant be prohibited. Since the fumigant market is the main market for methyl bromide, a need is apparent for tetrabromobisphenol-A processes which do not co-produce a substantial amount of methyl bromide. This is a difficult task since such processes, to be commercially successful, will be required to economically produce tetrabromobisphenol-A without the benefit of the revenue realized from the sale of the co-produced methyl bromide.
The Invention
The processes of this invention feature the efficient production of high-quality tetrabromobisphenol-A in high yields. The processes can be run in the batch mode or in the continuous mode. When run in the batch mode, process efficiency is enhanced due to relatively short reactor times as there is no need for a time consuming one hour plus post-reaction cook period or a post-reaction tetrabromobisphenol-A precipitation step. The use of a continuous process for the production of tetrabromobisphenol-A is unique in itself and is made possible by the short reaction and tetrabromobisphenol-A precipitation times which are features of processes of this invention. In the continuous mode, reactor size can be substantially reduced without a loss in product output.
In addition to the above reaction efficiencies, the processes of this invention are capable of producing high yields of tetrabromobisphenol-A in a methanol based solvent without the substantial concomitant production of methyl bromide, say as low as 1.0 to 0.2 lbs of methyl bromide per 100 lbs of tetrabromobisphenol-A product. Even further, it is possible to obtain high yields of tetrabromobisphenol-A even though only about 2 moles of Br.sub.2 per mole of bisphenol-A are fed to the reactor.
It has been discovered that the foregoing benefits can be obtained by (1) brominating bisphenol-A in the presence of a water miscible solvent, e.g., methanol, and a relatively large amount of water while maintaining the reaction mass at a relatively high temperature and, concurrent therewith, (2) oxidizing HBr produced in the reaction mass to Br.sub.2 for use in the bromination. As will be discussed later, the features in (1) have conventionally been considered conducive to the low-yield production of low-quality tetrabromobisphenol-A and/or the co-production of methyl bromide.
In accordance with this invention, tetrabromobisphenol-A can be produced by:
a. feeding, to a reactor, a solution comprised of bisphenol-A, water and a water miscible solvent to at least partially form a reaction mass having a liquid phase containing from above about 15 to about 65 wt % water, the wt % being based upon the amount of water and water miscible solvent in the liquid phase;
b. be during (a), providing for the presence of unreacted Br.sub.2 in the reaction mass to yield a tetrabromobisphenol-A precipitate;
c. oxidizing HBr produced in the reaction mass to yield Br.sub.2 ; and
d. having a reaction mass temperature which is within the range of from about 50.degree. to about 100.degree. C.
Also in accordance with this invention, tetrabromobisphenol-A can be produced by:
a. co-feeding Br.sub.2 and a solution comprised of bisphenol-A, water and a water miscible solvent to a reactor to at least partially form a reaction mass having a liquid phase and a solid phase, the liquid phase containing water in an amount of from above about 15 to about 65 wt % water, the wt % being based upon the amount of water and water miscible solvent in the liquid phase;
b. the reaction mass liquid phase containing at least about 50 ppm unreacted Br.sub.2 during (a);
c. oxidizing HBr produced in the reaction mass to yield Br.sub.2 ; and
d. having a reaction mass temperature within the range of from about 50.degree. to about 100.degree. C.
The formation of the reaction mass can best be accomplished by co-feeding the Br.sub.2 and bisphenol-A/water/solvent solution. By co-feeding, it is meant that the Br.sub.2 and solution feed periods overlap one another to at least some extent. (A feed period is that period of time over which all of a subject feed is fed to the reactor.) For example, the Br.sub.2 feed can be initiated and then followed by the solution feed, with both feeds thereafter occurring simultaneously until finished. Another example would be that of an initial Br.sub.2 feed followed by a continuous solution feed which is accompanied by a continued, but intermittently interrupted or staged, Br.sub.2 feed. Yet another example is that of initiating the Br.sub.2 feed followed by the solution feed so that the two feeds occur simultaneously until the specified amount of Br.sub.2 has been fed. At that point, the solution feed continues alone until it is finished. Other co-feed schemes could feature an intermittently interrupted solution feed, or initially feeding the solution into a Br.sub.2 containing reactor followed by a combined Br.sub.2 and solution feed. Finally, the Br.sub.2 and solution feeds can be, timewise, completely concurrent one with the other.
Feeds that do not have some overlap of the Br.sub.2 and solution feed periods are possible, but will not be generally preferred. For example, all of the Br.sub.2 can be fed followed by the solution feed. However, depending on reaction conditions, such a feed scheme could lead to the formation of undesirable by-products due to the high concentration of Br.sub.2 which is seen by the initial bisphenol-A feed. Another scheme, i.e., feeding large amounts of bisphenol-A before feeding Br.sub.2, would not be preferred as it could lead to precipitation of substantial amounts of tribromobisphenol-A.
However the feeding occurs, it must be in harmony with the requirements of step (b) of the process.
Commercially available Br.sub.2 is suitable for use as the Br.sub.2 feed. Should the Br.sub.2 contain undesirable impurities, it can be treated by conventional purification techniques, e.g., distillation, H.sub.2 SO.sub.4 treatment, etc., which are well known to those skilled in the art.
The Br.sub.2 can be fed as a liquid or as a gas to the reactor. It is preferred that the feed be gaseous. Whether the Br.sub.2 is liquid or gaseous, it is preferred that the feed entry point be sub-surface of the reaction mass. This is conveniently accomplished by use of a dip tube. If the feed is liquid, abovesurface feed must contend with possible splattering and inefficient mixing.
The amount of water in the reaction mass should be within the range of from above about 15 to about 65 wt % water based upon the total amount of water and water miscible solvent in the liquid phase of the reaction mass. Preferably, the amount of water fed is that amount which is within the range of from about 25 to about 65 wt % water. Most highly preferred is the range of from about 25 to about 50 wt %. When the water miscible solvent is methanol, the preferred amount of water is from about 30 wt % to about 45 wt %.
The water content of the reaction mass is an important aspect of this invention. It is believed, though the processes of this invention are not to be limited by any particular theory, that the water content greatly attenuates the formation of methyl bromide and, at the same time, allows for a high purity tetrabromobisphenol-A product.
The formation of methyl bromide is attenuated because HBr, which is co-produced by the substitution bromination reaction between bisphenol-A and Br.sub.2, is diluted by the large amount of water in the reaction mass. Further, the HBr reacts with the water to yield H.sub.3 OBr which is very slow to react with a water miscible solvent, e.g., methanol.
In view of the amount of water present, the tetrabromobisphenol-A product purity is unexpected. Normally, it would be expected that this amount of water would cause under-brominated species, e.g., tribromobisphenol-A, to precipitate along with the tetrabromobisphenol-A species. This co-precipitation would be contrary to the obtainment of a highly pure tetrabrominated product. Not only does the large amount of water not act detrimentally towards the processes of this invention, but, instead, it benefits the tetrabromination reaction. Without being limited to any particular theory, it is believed that the water enhances the presence of brominating species in the reaction mass. With this enhancement, there is a favoring of the bromination of the bisphenol-A all of the way to tetrabromobisphenol-A before the intermediate tribromobisphenol-A has sufficient opportunity to form a precipitate. It is believed that the enhancement of the brominating species is due to the fact that HBr reacts with water to form the H.sub.3 OBr acid. The H.sub.3 OBr acid does not react with Br.sub.2. This is important because if H.sub.3 OBr was not formed, a larger quantity of HBr would be available to react with Br.sub.2 to form HBr.sub.3. The formation of HBr.sub.3 is not desired as it is a non-brominating species in the reaction mass. Thus, the formation of HBr.sub.3 consumes reaction mass Br.sub.2 which in turn results in a slowing of the bromination reaction. This slowing of the bromination reaction can result in an increase in the precipitation of tribromobisphenol-A.
The water content is not the only factor affecting the quantity of HBr in the reaction mass. The HBr quantity can also be reduced by reacting the HBr with an oxidant in accordance with this invention. As will be discussed later, the use of an oxidant will convert at least some of the HBr to Br.sub.2. Thus, the large amount of water and the use of an oxidant can both contribute to enhancing the presence of brominating species in the reaction mass.
The water being fed to the reactor has heretofore been described as being part of a solution which also contains bisphenol-A and a water miscible solvent. Feeding the water as part of such a solution is convenient and preferred. However, the water may be introduced into the reaction mass in other equivalent ways. For example, the water can be fed as a separate feed stream. Such a feed could be essentially contemporaneous with the feed of a solution of bisphenol-A and water miscible solvent. Even further, a portion, if not all, of the water can be fed as steam or steam condensate along with a gaseous Br.sub.2 feed. The steam could have been used to vaporize the Br.sub.2 to form the gaseous feed. Another example features providing water as a charge or as part of a charge to the reactor prior to initiating the feeds and adjusting the amount of water later fed to obtain the desired water content in the reaction mass. However the water is provided to the reaction mass, the only requirement for the water feed is that it be such that the proper amount of water be present in the reaction mass during substantially all of the reaction period.
In those cases where the amount of water used is in the lower end of the range, say 15 to 25 wt %, it may be desirable to add some additional water at the end of the bisphenol-A bromination. The possible advantage to such an addition is that the additional water may cause further precipitation of tetrabromobisphenol-A from the reaction mass. The further precipitation goes towards increasing the yield of the process. In these cases, the added water is counted in the total solution water.
The feed of the water miscible solvent has been described above in conjunction with the feed of the solution. However, the solvent feed need not always be exclusively as a constituent of the solution provided that the solvent's functions are not hindered. For example, a portion of the solvent can be fed as part of the solution as is needed to solvate the bisphenol-A in the solution, while the remaining portion, generally a smaller portion, can be fed as a separate stream. From a practical standpoint though, the solvent is best fed as a solution constituent.
As can be appreciated from the foregoing, the manner in which the water, solvent and/or solution can be fed is not critical to the processes of this invention provided that the reaction mass is properly constituted. Thus, to simplify matters for discussion, the feed of the solution, which comprises bisphenol-A, water and water miscible solvent, is to mean that the water can be fed as a constituent of the solution, as a separate stream or as a combination of both and that the solvent can all be fed as a constituent of the solution or as a portion in the solution and as a portion in a separate stream. Also to be considered as part of the solution feed is any water or solvent which is provided to the reaction mass as a pre-feed charge or as a part of such a charge to the reactor.
The water miscible solvent can be defined functionally as a material which is capable of solvating Br.sub.2, bisphenol-A, monobromobisphenol-A, dibromobisphenol-A and tribromobisphenol-A under reaction conditions. The reaction conditions of special import are the reaction mass temperature, the presence of unreacted Br.sub.2 in the reaction mass and the reaction mass water content. Further, the solvent should be substantially inert with regard to H.sub.3 OBr and the ar-bromination of the bisphenol-A to tetrabromobisphenol-A and not contribute to the production of troublesome amounts of color bodies, ionic bromides and/or hydrolyzable bromides. Hydrolyzable bromides include 1-bromo-2-methoxy-2-(3',5'-dibromo-4'-hydroxyphenyl)propane, 1,1-dibromo-2-methoxy-2-(3',5'-dibromo-4'-hydroxyphenyl)propane, 1,3-dibromo-2-methoxy-2-(3',5'-dibromo-4'-hydroxyphenyl)propane, and 1,1,3-tribromo-2-methoxy-2-(3',5'-dibromo-4'-hydroxyphenyl)propane. The solvent, when taken in combination with the water and reaction conditions of the processes of this invention, can have some small ability to solvate tetrabromobisphenol-A, but for the sake of reaction yield, the solvating power should be low, say no more than about 20 wt % and preferably no more than about 5 wt % solvated tetrabromobisphenol-A in the liquid phase of the reaction mass.
Exemplary of the preferred water miscible solvents are water miscible alcohols, carboxylic acids, e.g., acetic acid, and nitriles, e.g., acetonitrile. Some ethers may also be suitable provided they are not cleaved by the acidic nature of the reaction mass. The more preferred solvents are the alcohols having up to 4 carbon atoms. Most preferred are ethanol and methanol, with methanol being the solvent of choice. Methanol is relatively inexpensive and is easily recovered by simple distillation techniques for recycle. Since there is a large water presence in the processes of this invention, it is not necessary to recover the methanol with a low water content, thereby reducing the methanol recovery cost.
When methanol is not used as the water miscible solvent, the co-production of methyl bromide is obviously not of concern. However, if the product between the HBr and the solvent is not a commercially desirable product, its production is not wanted. Thus, the features of the process of this invention are beneficial whether or not the solvent is methanol.
The amount of water miscible solvent used is best related to the amount of bisphenol-A fed and can be conveniently expressed as the weight ratio of the solvent to bisphenol-A. Preferably, the ratio is within the range of from about 2:1 to about 10:1, and most preferably within the range of from about 3:1 to about 5:1. More or less solvent can be used, provided that the solvent function mentioned above is accomplished.
The Br.sub.2 and solution feed streams are preferably at a temperature which promotes process efficiency in view of the desired reaction mass temperature. A suitable liquid Br.sub.2 feed temperature is from about 10.degree. C. to just below the boiling point of Br.sub.2. If the Br.sub.2 is to be fed as a gas, then the Br.sub.2 stream temperature should be that which is conducive to such a feed. For example, such a feed temperature may be within the range of from about 60 to about 100.degree. C. The solution feed temperature should be that which does not detrimentally cool or heat the reaction mass or which requires pressure operation so that the feed can be made in the liquid state. If the solution feed is to be made with separate water and/or solvent feeds, then the same comments made above with regard to temperature apply to the separate feeds.
The Br.sub.2 and solution and/or separate water, etc., feeds all contribute to the formation of the reaction mass in the reactor. These feeds will produce a reaction mass liquid phase (liquid portion) and, because of the formation of tetrabromobisphenol-A precipitate, ultimately a reaction mass solid phase (solid portion). At least a portion of the Br.sub.2 feed, be it fed as a gas or as a liquid, will be consumed in the bromination reaction. Any non-consumed Br.sub.2 feed will be found in the liquid phase and be joined there by any non-consumed Br.sub.2 produced by the oxidation of HBr present in the reaction mass. While the identity of the source of the unreacted Br.sub.2 in the liquid phase is lost, the combination of non-consumed Br.sub.2 from the feed and from the oxidized HBr provides for the excess of unreacted Br.sub.2 in the liquid phase which is a feature of this invention.
The unreacted Br.sub.2 in the liquid phase of the reaction mass is extant as the solution is being fed. It is permissible for the unreacted Br.sub.2 content in the reaction mass to disappear for brief periods of time depending on the level of under-brominated species that can be tolerated in the tetrabromobisphenol-A reaction product and/or upon the extent of precipitation of the under-brominated species which is realized. In fact, if the period of time is real brief and favorable reaction parameters are chosen, the formation of these under-brominated precipitates may not occur to any appreciable extent at all. The practitioner will have to observe the process and determine by empirical methods the sensitivity of the chosen reaction conditions to the brief absence of unreacted Br.sub.2 in the reaction mass. Thus, for the purposes of this invention the "presence of unreacted Br.sub.2 " encompasses brief periods of time in which the unreacted Br.sub.2 content can be nil, but which does not result in the formation of under-brominated species to an extent that results in an unacceptable tetrabromobisphenol-A product, say one containing less that about 96 wt % tetrabromobisphenol-A.
Quantifying the preferred amount of unreacted Br.sub.2 in the reaction mass liquid phase is best handled by a trial and error technique. A trial process is first defined by choosing an unreacted Br.sub.2 level and the other process parameters. The produced tetrabromobisphenol-A product from the process is analyzed for its tri- and tetrabromobisphenol-A content. If the tribromobisphenol-A level is too high, another trial process is constructed with a higher unreacted Br.sub.2 level. The procedure is repeated until the desired product is obtained. (Note that some benefit towards reducing the tribromobisphenol-A content can also be obtained by using a higher reaction temperature.) As the chosen unreacted Br.sub.2 content gets higher, care should be taken that the unreacted Br.sub.2 content will not be so high that it results in the production of tribromophenol and other by-products which are not desirable from a commercial standpoint.
Measuring the unreacted Br.sub.2 content of the reaction mass can be performed by the use of colorimetric techniques. A useful technique comprises the formation of an acidic (HBr) water and methanol solution. From this solution, several standard samples are prepared, to each of which is added a different and measured amount of Br.sub.2. The colors of these sample solutions are then compared colorimetrically with the color of the liquid of phase of the reaction mass. A color match is indicative of the amount of Br.sub.2 in the liquid phase. Colorimetric determination for unreacted Br.sub.2 is quite suitable as unreacted Br.sub.2 colors the sample solutions and the reaction mass in accordance with its concentration. Low concentrations give a pale yellow color; intermediate concentrations give a strong yellow color; high concentrations give an orange color; and the highest concentrations give a dark red color. Unreacted Br.sub.2 concentrations in excess of 50 ppm and preferably within the range of from about 100 to about 10,000 ppm, based upon the weight of the reaction mass liquid portion are suitable. A more preferred amount is within the range of from about 100 to 5000 ppm, with the most preferred amount being within the range of from about 200 to 2000 ppm.
The unreacted Br.sub.2 concentrations are maintained in the reaction mass so long as bisphenol-A and under-brominated species are likewise present. As can be appreciated, the unreacted Br.sub.2 content diminishes as the Br.sub.2 reacts, thus, the Br.sub.2 feed acts to replenish the Br.sub.2 in the reaction mass. Using the above-described colorimetric technique, the practitioner can monitor the unreacted Br.sub.2 content of the reaction mass during the process and keep the unreacted Br.sub.2 content within the chosen target range by adjusting the Br.sub.2 feed, the solution feed or both. Since there will be tetrabromobisphenol-A precipitate in the reaction mass, colorimetric monitoring may require that a small stream be taken from the reactor and filtered to remove the solids before being submitted to a colorimetric technique. It may also be possible to read the intensity of the reaction mass color without filtration by the use of reflectance techniques which measure the intensity of the light reflected off of the reaction mass. In all of the colorimetric cases, the color of the liquid phase of the reaction mass is the determinative factor.
It is to be understood that techniques other that colorimetric techniques may be used in monitoring and obtaining the desired unreacted Br.sub.2 level in the reaction mass. Though the particular technique used is not critical to the processes of this invention, the use of the colorimetric technique is highly preferred.
It is also to be understood that the method used to obtain the desired unreacted Br.sub.2 level can be by a method other than the adjustment of the before-mentioned feeds. For example, when an oxidant is used to convert HBr to Br.sub.2, the amount of Br.sub.2 generated can be regulated by controlling the amount of oxidant fed to the reaction mass. The amount of unreacted Br.sub.2 contributed to the reaction mass by oxidation of HBr can be substantial considering that four moles of HBr are generated for each mole of tetrabromobisphenol-A produced. Thus, when additional Br.sub.2 is needed, the practitioner can use the oxidation of HBr to generate at least a part of the Br.sub.2 needed to obtain the desired unreacted Br.sub.2 level.
With the use of an oxidant to oxidize the HBr to Br.sub.2, the processes of this invention can obtain good results by feeding only about two moles or slightly more of Br.sub.2 to the reactor for every one mole of bisphenol-A fed. The other two moles of Br.sub.2 that are needed are provided by the full oxidation of the cogenerated HBr. If there is less than full HBr oxidation, then the amount of Br.sub.2 fed to the reactor will be that amount, in sum with the Br.sub.2 formed by oxidation, which will provide at least stoichiometric quantities of Br.sub.2 and preferably quantities which are in slight excess of stoichiometric, say from about 0.1% to about 3% percent of stoichiometric. Stoichiometric Br.sub.2 for the ar-tetrabromination of bisphenol-A is four moles of Br.sub.2 per mole of bisphenol-A. As can be appreciated, if the oxidation of HBr is not part of process, then the Br.sub.2 feed would be at least stoichiometric (four moles of Br.sub.2 per mole of bisphenol-A fed), with the slight excess being preferred.
Irrespective of the Br.sub.2 source, the slight stoichiometric excess is desirable since it is less difficult to control the process by having excess Br.sub.2 present at least during most of the reaction period. For batch processes, the excess Br.sub.2 present after completion of the process can be removed by treating the reaction mass with a reducing agent such as sodium sulfite or hydrazine.
The oxidant material is any oxidant which is capable of oxidizing HBr to Br.sub.2 in the reaction masses and under the process conditions of this invention. Preferred oxidants are those in liquid form which can facilitate their feed to the reactor. Preferred oxidants are chlorine and hydrogen peroxide.
When Cl.sub.2 is the oxidant, it can be fed to the reaction mass as a gas or as a liquid. The gaseous feed is preferred. To mitigate against the formation of chlorinated bisphenol-A, it is preferred that the Cl.sub.2 be fed after initiation of the Br.sub.2 feed. After the initial Br.sub.2 feed, Cl.sub.2 can be fed contemporaneously with the Br.sub.2 feed. Even with this feed sequence, some bromochlorobisphenol-A compounds will be formed. Fortunately, these bromochloro species are present in very minor amounts, say from about 50 to about 500 ppm, based on the total weight of the precipitate. The most predominate bromochloro specie will, in most cases, be chlorotribromobisphenol-A.
It is theorized that the low amount of bromochloro species is due to the fact that the bromination of the bisphenol-A to tetrabromobisphenol-A occurs rapidly. Thus, there is never a large enough concentration of under-brominated species, e.g., tribromobisphenol-A, in the reaction mass with which the Cl.sub.2 can react in preference to reacting with the HBr.
When the oxidant is H.sub.2 O.sub.2, safety makes it preferably that it be fed to the reaction mass in an aqueous solution containing no more than about 90 wt % H.sub.2 O.sub.2. Preferred are aqueous solutions containing from about 30 to about 80 wt % H.sub.2 O.sub.2. A most preferred solution is one containing from about 50 to about 70 wt % H.sub.2 O.sub.2.
The H.sub.2 O.sub.2 can be fed to the reaction mass at any time. For batch operation, it is preferred that the H.sub.2 O.sub.2 be fed after most of the Br.sub.2, say above about 50%, has been fed. For continuous operation, the H.sub.2 O.sub.2 feed would most preferably occur contemporaneously with at least most of the BF.sub.2 feed. Most preferably, the H.sub.2 O.sub.2 feed would start after initiating the Br.sub.2 feed.
The oxidants can be fed to the reaction mass separately or in some cases, along with the Br.sub.2 feed. It is preferred that the Cl.sub.2 be fed through the same feed conduit as is the Br.sub.2 and may be fed while Br.sub.2 is being fed. In distinction, the H.sub.2 O.sub.2 is preferably fed as a separate feed stream.
The amount of oxidant fed is preferably that amount needed to maximize the amount of HBr oxidized without leaving a large excess of oxidant present in the reaction mass. Assuming that one mole of the oxidant chosen will oxidize two moles of HBr, the mole ratio of oxidant to bisphenol-A fed should be within the range of from about 1:1 to about 2:1. A more preferred mole ratio is from about 1.5:1 to about 1.9:1. The higher oxidant ratios are preferred when H.sub.2 O.sub.2 is the oxidant, while the mid range ratios, say 1.5-1.8:1 are preferred when Cl.sub.2 is the oxidant. The reason that the lower oxidant ratios are preferred for Cl.sub.2 is that there is a balance between the amount of HBr oxidized and the amount of chlorobromo species which can be tolerated. If there is no need to keep the chlorobromo species to some minimum amount, then more Cl.sub.2 is permissible. Adjustments to the above ranges are necessary if the oxidant chosen does not oxidize the HBr on a one to two basis. In these cases, the ranges are adjusted in proportion to the variance in the one to two relationship.
Another important consideration in practicing the processes of this invention is the reaction mass temperature during the bromination period. It is desirable to use a relatively high temperature so that the bromination of the bisphenol-A to tetrabromobisphenol-A will be sufficiently fast to attenuate the formation of tribromobisphenol-A precipitate. However, there is a practical limit as to how high the temperature can be. For example, the practitioner would not want to use temperatures which would cause the production of unacceptable levels of unwanted by-products or the degradation of the tetrabromobisphenol-A product.
It is unusual to operate a tetrabromobisphenol-A process at relatively high temperatures. This is especially so when the production of a co-product, e.g., methyl bromide, is to be minimized as it is conventional to expect that high temperatures will yield large amounts of methyl bromide. Also, the use of high temperatures is not conventional when the precipitation of the tetrabromobisphenol-A is to occur under reaction conditions soon after it is formed-such precipitation being a feature of the processes of this invention. It would be expected that high temperatures would frustrate such precipitation by increasing the solubility of the tetrabromobisphenol-A in the solvent solution and require a final cooling of or addition of water to the reaction mass to effect the desired precipitation. The processes of this invention are not so affected, nor is there required a cooling step to obtain tetrabromobisphenol-A precipitation.
Not only do the high temperatures of this invention contravene conventional tetrabromobisphenol-A thinking, but also such temperatures have been found to provide for process economy and product purity. Process economy, in part, is realized because, with higher reaction mass temperatures, the process of this invention can use cooling tower water to cool the reactor instead of having to use refrigeration which is required by the low temperature processes.
Preferred temperatures are within the range of from about 30.degree. to about 100.degree. C. More highly preferred temperatures are within the range of from about 50.degree. to about 80.degree. C. The most highly preferred temperatures are within the range of from about 50.degree. to about 70.degree. C. Temperatures below 30.degree. C. can be used, but the solvent to bisphenol-A weight ratio may well need to be high, say from 8:1 to 15:1. For these ratios, temperatures of 30.degree. to 50.degree. C. may be suitable.
The bromination of bisphenol-A is an exothermic reaction as is the oxidation of HBr with H.sub.2 O.sub.2. To control the reaction mass temperature, it may become necessary to remove heat from the reaction mass. Heat removal can be effected by running the reaction at reflux with the condenser facilitating the heat removal. If it is desired to operate at a temperature below the atmospheric boiling point of the reaction mixture, the reaction can be run under sub-atmospheric pressure.
Generally, the basic concepts of the processes of this invention are not appreciably affected by the process pressure. Thus, the process can be run under sub-atmospheric, atmospheric or super-atmospheric pressure.
At process initiation, it is desirable to charge the reactor with a liquid pre-reaction charge which will become a part of the reaction mass upon the commencement of the feed. The liquid charge will provide a stirtable reaction mass and act as a heat sink to moderate temperature changes in the reaction mass. The liquid charge is preferably comprised of water and the same water miscible solvent fed in the solution. It is preferred that the liquid charge be acidic, e.g., containing from 1 to 20 wt % acid such as HBr, HCl, or the like. The acid seems to promote good color in the initial tetrabromobisphenol-A produced. Further, it is preferred that the solvent be saturated with solvated tetrabromobisphenol-A. It is also preferred that the reactor be charged with seed particles of tetrabromobisphenol-A. The saturation of the solvent and the presence of the seed particles both act to enhance the precipitation of the tetrabromobisphenol-A produced during the bromination period. It is most practical to use a heel from a previously run process of this invention as the liquid charge. The tetrabromobisphenol-A seed particles can be brought over from the previous run or can be added separately. If a heel is not available, it is also possible to use a separate water and water miscible solvent feed, which are a part of the total solution feed, to form the liquid charge. In this scheme, an initial amount of water and water miscible solvent are fed to the reactor prior to the initiation of the solvated bisphenol-A portion of the solution feed. The only caveat to this scheme is that there must be apportionment of the various feeds making up the solution feed so that there will still be compliance with the various parameters which define the processes of this invention.
If the process of this invention is run as a batch process, the Br.sub.2 and solution feeds are fed to a stirred reactor until they are exhausted. There is no need for a post-feed cook period of any significant length as, under the reaction conditions, the bromination of bisphenol-A to tetrabromobisphenol-A occurs quite rapidly. Also, since the water content of the reaction mass is so large and since the tetrabromobisphenol-A is so insoluble in the presence of such an amount of water, there is only a modicum of benefit in cooling the final reaction mass. The benefit of cooling resides mainly in reducing the vapor pressure of solvated gaseous bromides, e.g., methyl bromide, in the reaction mass prior to the liquid-solids separation. There also could be some slowing of the formation of these bromides. In addition, depending upon the water content of the reaction mass, cooling may allow for additional precipitation of tetrabromobisphenol-A from the reaction mass. When operating within the preferred ranges recited herein, the additional precipitation benefit may not be worth the cost associated to obtain same. Finally, depending on the separation technique used, cooling the reaction mass may make it easier to handle downstream from the reactor. Thus, if none of the above are of concern or relative value, then the reaction mass can be subjected to liquid-solids separation as soon as it can be transported to the separation equipment. If, however, cooling is desired, the cooling time will depend upon how the reaction mass is to be cooled and to what temperature it is to be cooled. In a laboratory setting, cooling times can range from about one to about thirty minutes.
Additional time may also be used between the end of the co-feed and the precipitate recovery, if it is desired, to add additional water to the reaction mass at the end of the co-feed to insure that even more tetrabromobisphenol-A precipitate is formed in the reaction mass. The water addition and precipitation time can be very short, say less than about thirty minutes.
Irrespective of whether or not the reaction mass is cooled or treated with more water, it is to be understood that the additional time used does not appreciably increase the total amount of tetrabromobisphenol-A produced by the process (the total amount includes that which is a precipitate and that which is solvated in the reaction mass). These additional times, therefore, are not to be considered cook times in the same way as are the cook times taught by the prior art processes.
After the recovery of the solids from the liquid, the solids are preferably washed with a solution of water and the particular water miscible solvent used in the reaction. The washing removes essentially all the mother liquor from the solids. The mother liquor contains impurities such as tribromophenol, HBr, and hydrolyzable impurities. A typical wash can be a 30 wt % methanol in water solution. The washed solids are then rewashed with deionized water to remove any remaining water miscible solvent from the first wash so as to minimize emission problems when drying the product.
When run in the continuous mode, the reactor is preferably a continuously stirred tank reactor. The reaction mass is being continuously formed and a portion thereof is being removed from the reactor during the reaction mass formation. The reactor design should be such that the average residence time in the reactor is sufficient to insure the tetrabromination of substantially all of the bisphenol-A. The terms "continuous feed" and "continuous withdrawal" are not meant to exclude interrupted feeds or withdrawals. Generally, such interruptions are of short duration and may be suitable depending upon the scale and design of the reactor. For example, since the tetrabromobisphenol-A precipitate will tend to settle near the bottom of the reactor, a withdrawal may be made and then stopped for a period of time to allow for precipitate build-up to occur prior to the next withdrawal. Such a withdrawal is to be considered continuous in the sense that the withdrawal does not await the completion of the reactor feeds as is characteristic of batch processes.
Whether the continuous withdrawal is interrupted or not, the withdrawal results in a portion of the liquid and a portion of the solids in the reaction mass to be withdrawn together. The solids portion will be predominately tetrabromobisphenol-A. This mix can be filtered, the precipitate washed, etc., as is done for the above described batch mode case.
When using the continuous mode of operation, it is believed that it would be beneficial if the reaction mass temperature be kept fairly high as compared to the temperatures preferred for the batch mode. Preferred batch mode temperatures are from about 50.degree. to about 65.degree. C. For the continuous mode, the preferred temperatures are within the range of from about 55.degree. to about 95.degree. C., and most preferably within the range of from about 65.degree. to about 95.degree. C. Very good results are predicted with temperatures of from about 65.degree. to about 75.degree. C. By using the higher temperatures, it was found that higher purity product could be obtained.
The benefit of high temperatures on product purity is understood in view of studies which support the correlation between product purity and the relative rates of bromination and precipitation of the tribromobisphenol-A intermediate. Raising the temperature benefits both the reaction rate and the solubility of the tribromobisphenol-A in the reaction mass liquid phase and thus, promotes the obtainment of a pure product. An increase in Br.sub.2 or an increase in the tribromobisphenol-A concentration in the liquid phase by reducing the liquid phase water content can also increase the bromination rate of the tribromobisphenol-A, but, they both present problems of their own. A high Br.sub.2 concentration can cause the formation of undesirable byproducts, while decreasing the liquid phase water content will increase the HBr content of the reaction mass and reduce tetrabromobisphenol-A yields.
It is expected that in the continuous mode of operation, that the preferred reactor residence time should be within the range of from about 30 to about 150 minutes when using a stirred-tank reactor and the process conditions which are preferred for that operating mode. Reactor residence time, as used here, is the reactor volume divided by the flow rate at which slurry is removed from the reactor.
The tetrabromobisphenol-A product produced by the processes of this invention can have a very high purity--say at least 98 wt % tetrabromobisphenol-A. The tribromobisphenol-A content is low--say from about 0.1 to about 2 wt %. The product quality is excellent, having an APHA color less than about 50 (80 grams of tetrabromobisphenol-A in 100 ml of acetone). Hydrolyzable bromides are also kept low, generally below about 60 ppm. The process yields are impressive, with yields within the range of from about 95 to about 99% being possible.
As can be appreciated from the foregoing, the water content of the solvent, the reaction temperature and the Br.sub.2 content in the reaction mass during the bisphenol-A feed all contribute to obtaining the desired tetrabromobisphenol-A product in an efficient manner. The selection of particular values for each of these process parameters to obtain the results desired will depend on each practitioners needs and upon the equipment available. One practitioner may emphasize one benefit of using a process of this invention over other possible benefits. Thus, that practitioner may select different process parameter values than those selected by another practitioner who desires to highlight other benefit(s).
The use of the oxidation of the co-generated HBr to produce a part of the Br.sub.2 needs for the processes of this invention is particularly attractive in those cases where the oxidation is more economical than the cost of providing for an equivalent amount of Br.sub.2 in the feed to the reactor. The economic advantage is usually extant in those cases where the costs of feeding four moles of Br.sub.2 minus the value of recovered HBr is greater than the costs of feeding two moles of Br.sub.2 plus the oxidation of the HBr.
Though preferably designed to minimize the production of methyl bromide, the processes of this invention are sufficiently adaptable to be modified to produce moderate amounts of methyl bromide--say 20 lbs per 100 lbs of tetrabromobisphenol-A product. In this way, a future market need, even though greatly reduced, can be accommodated. When there is a production of methyl bromide, the total Br.sub.2 requirements of the process will be those amounts needed to produce the tetrabromobisphenol-A in a high yield and to produce the targeted amount of HBr. In these cases, the Br.sub.2 feed and the amount of Br.sub.2 generated from oxidation must be sufficient together to meet the four Br.sub.2 requirements.
While the foregoing descriptions of the oxidation of HBr generally speak of the HBr being oxidized in the reactor or reaction mass, it is within the scope of the processes of this invention to also remove co-produced HBr from the reactor and oxidize it outside of the reactor and then to send the so produced Br.sub.2 back to the reactor.
It is also within the scope of the processes of this invention to provide HBr to the reactor from a source other than the reaction in the reactor. This non-indigenous HBr can be oxidized along with the co-generated HBr to yield Br.sub.2. The Br.sub.2 produced from the non-indigenous HBr can then count against the total Br.sub.2 needs of the process and the appropriate adjustment in the Br.sub.2 feed can be made.